Introduce arm_acle.h supporting existing LLVM builtin intrinsics

Summary: This patch introduces ACLE header file, implementing extensions that can be directly mapped to existing Clang intrinsics. It implements for both AArch32 and AArch64.

Reviewers: t.p.northover, compnerd, rengolin

Reviewed By: compnerd, rengolin

Subscribers: rnk, echristo, compnerd, aemerson, mroth, cfe-commits

Differential Revision: http://reviews.llvm.org/D4296